How to find the supervisor of an OTP process? - erlang

Are there functions which would allow an OTP process to find the pid of its supervisor?

The data is hidden in the process dictionary (of any process spawned with proc_lib) under the entry '$ancestors':
1> proc_lib:spawn(fun() -> timer:sleep(infinity) end).
<0.33.0>
2> i(0,33,0).
[{current_function,{timer,sleep,1}},
{initial_call,{proc_lib,init_p,3}},
{status,waiting},
{message_queue_len,0},
{messages,[]},
{links,[]},
{dictionary,[{'$ancestors',[<0.31.0>]},
{'$initial_call',{erl_eval,'-expr/5-fun-1-',0}}]},
{trap_exit,false},
{error_handler,error_handler},
{priority,normal},
{group_leader,<0.24.0>},
{total_heap_size,233},
{heap_size,233},
{stack_size,6},
{reductions,62},
{garbage_collection,[{min_bin_vheap_size,46368},
{min_heap_size,233},
{fullsweep_after,65535},
{minor_gcs,0}]},
{suspending,[]}]
Here the line that interests us is {dictionary,[{'$ancestors',[<0.31.0>]},.
Note that this is the kind of stuff you should rarely have any reason to use yourself. As far as I know, it's mostly used to handle clean termination in supervision trees rather than introspection for whatever code you have. Handle with care.
A cleaner way to do things without messing with OTP's sensible innards would be to have the supervisor pass its own pid as an argument to the process when starting it. This should be far less confusing for the people who'll read your code.

If you want to do it wrong, here's our solution:
%% #spec get_ancestors(proc()) -> [proc()]
%% #doc Find the supervisor for a process by introspection of proc_lib
%% $ancestors (WARNING: relies on an implementation detail of OTP).
get_ancestors(Pid) when is_pid(Pid) ->
case erlang:process_info(Pid, dictionary) of
{dictionary, D} ->
ancestors_from_dict(D);
_ ->
[]
end;
get_ancestors(undefined) ->
[];
get_ancestors(Name) when is_atom(Name) ->
get_ancestors(whereis(Name)).
ancestors_from_dict([]) ->
[];
ancestors_from_dict([{'$ancestors', Ancestors} | _Rest]) ->
Ancestors;
ancestors_from_dict([_Head | Rest]) ->
ancestors_from_dict(Rest).

Related

Erlang: How to properly dispatch a gen_server with start_child in a supervisor and call the API

I have a gen_server in my cavv application that I need to start first to execute a call to. I want to use a command dispatcher for this. For a short example, this it the gen_server's API:
a gen_server: cavv_user
-module(cavv_user).
-behavior(gen_server).
-define(SERVER(UserId), {via, gproc, {n, l, {?MODULE, UserId}}}).
start_link(UserId) ->
gen_server:start_link(?SERVER(UserId), ?MODULE, [UserId], []).
change_email_address(UserId, EmailAddress) ->
gen_server:call(?SERVER(AggregateId), {execute_command, #change_user_email_address{user_id=UserId, email_address=EmailAddress}}).
Before I can call cavv_user:change_email_address(). I need to start the cavv_user. I do this is as a simple_one_for_one child in a supervisor, like so:
a supervisor: cavv_user_sup
-module(cavv_user_sup).
-behaviour(supervisor).
-define(CHILD(ChildName, Type, Args), {ChildName, {ChildName, start_link, Args}, temporary, 5000, Type, [ChildName]}).
start_link() ->
supervisor:start_link({local, ?SERVER}, ?MODULE, []).
start_child(UserId) ->
supervisor:start_child(?SERVER, [UserId]).
init([]) ->
RestartStrategy = {simple_one_for_one, 1, 5},
Children = [?CHILD(cavv_user, worker, [])],
{ok, { RestartStrategy, Children} }.
The problem I am now facing is how to dispatch commands to a cavv_user. I want to make sure the proper user is started first using start_child, and then call the cavv_user:change_email_address().
I have found this anwser, to use a dispatcher: Erlang: what supervision tree should I end with writing a task scheduler?
So I created a command dispatcher and end up with a cavv_user_dispatcher and a cavv_user_dispatcher_sup that in turn contains the cavv_user_dispatcher and the earlier cavv_user_sup:
cavv_user_dispatch_sup
| |
cavv_user_dispatcher |
(gen_server) |
|
|
cavv_user_sup
| | |
cavv_user_1...cavv_user_N
The cavv_user_dispatcher
This works beautifully.
The problem I am facing now is, how do I properly write the code in cavv_user_dispatcher? I am facing a problem with code duplication. How to properly call start_child and call the appropriate API of cavv_user?
Should I use some kind of Fun like so?
-module(cavv_user_dispatcher).
dispatch_command(UserId, Fun) ->
gen_server:call(?SERVER, {dispatch_command, {UserId, Fun}}).
handle_call({dispatch_command, {UserId, Fun}}, _From, State) ->
cavv_user_sup:start_child(UserId),
Fun(), %% How to pass: cavv_user:change_email_address(..,..)?
{reply, ok, State};
Or duplicate the cavv_user's API like so?
-module(cavv_user_dispatcher).
change_user_email_address(UserId, EmailAddress) ->
gen_server:call(?SERVER, {change_user_email_address, {UserId, EmailAddress}}).
handle_call({change_user_email_address, {UserId, EmailAddress}}, _From, State) ->
cavv_user_sup:start_child(UserId),
cavv_user:change_email_address(UserId, EmailAddress),
{reply, ok, State};
Or should I re-use the command records from cavv_user into some kind of util to properly build them and pass them around? Maybe some better way to pass the function I want to call at cavv_user?
I would like to solve the problem in the best Erlang way as possible, without code duplication.
Is your dispatcher supposed to handle other commands?
If yes then then how will the next command will come, I mean will the requester know the process pid of the user or not?
if yes then you need 2 functions, one to create a user, it will return the pid to the requester for next call, and one to handle next requests by sending the command to the given pid
if no, then you need also 2 functions, one to create the a user and store the user_id along with the user process pid and one to handle next request by retrieving the process pid and then forward it the command (I suppose this is what you want to do).
if no then you don't need to handle any command and should pass directly the email address when creating the user process. Note that this is true for all cases since you need a different interface to create a user.
I would modify your code this way (not tested, it is too late :o) !)
-module(cavv_user_dispatcher).
create_user(UserId,UserMail) ->
gen_server:call(?SERVER,{new_user,UserId,UserMail}).
% Args is a list of argument, empty if
% F needs only one argument (the user Pid)
dispatch_command(UserId, Fun, Args) ->
gen_server:call(?SERVER, {dispatch_command, {UserId, Fun,Args}}).
handle_call({dispatch_command, {UserId, Fun,Args}}, _From, State) ->
Pid = get_pid(UserId,State),
Answer = case Pid of
unknown_user_id -> unknown_user_id;
_ -> apply(Fun,[Pid|Args]),
ok
end,
{reply, Answer, State};
handle_call({new_user,UserId,UserMail},_From,State) ->
% verify that the user id does not already exists
CheckId = check_id(UserId,State),
{Answer,NewState} = case CheckId of
false -> {already_exist,State};
true -> {ok,Pid} = cavv_user_sup:start_child(UserId,UserMail)
{ok,[{UserId,Pid}|State]}
% State must be initialized as an empty list in the init function.
{reply, Answer, NewState};
...
get_pid(UserId,State) ->
proplists:get_value(UserId, State, unknown_user_id).
check_id(UserId,State) ->
not proplists:is_defined(UserId, State).
and the user supervisor mus be modified this way:
start_child(UserId,UserMail) -> % change arity in the export
supervisor:start_child(?SERVER, [UserId,UserMail]).
and then the user server:
start_link(UserId,UserMail) ->
gen_server:start_link(?SERVER(UserId), ?MODULE, [UserId,UserMail],[]).
init([UserId,UserMail]) ->
{ok,[{user_id,UserId},{user_mail,UserMail}]}.

Should I not call gen_server:stop() directly?

In the LYSE book the author handles the termination of the server as follows:
%% Synchronous call
close_shop(Pid) -> gen_server:call(Pid, terminate).
handle_call(terminate, _From, Cats) ->
{stop, normal, ok, Cats}.
terminate(normal, Cats) ->
[io:format("~p was set free.~n",[C#cat.name]) || C <- Cats],
ok.
So it returns a stop value from the handle_call callback.
Here is how I wrote it:
close_shop(Pid) -> gen_server:stop(Pid).
terminate(_Reason, {Cats, Money}) ->
io:format("Made $~w~n", [Money]),
[io:format("~p was set free.~n",[C#cat.name]) || C <- Cats].
Is this not a good practice then to call gen_server:stop() directly?
It is not a bad practice to call gen_server:stop/1,3 directly. It does an almost same thing as the example from LYSE but without calling handle_call/3 from your module. Try and check it out. You can even read the source code to be sure.

How to find where a process started in elixir

I am using observer in elixir and the following is the snapshot of an Application [under applications tab]:
I need to exit these processes once their work is done. Somehow, I am not able to figure out where some of the processes are originating. Is there a way in elixir/erlang to figure out the module/function where a particular process was created?
Suggestions will be highly appreciated. Thanks.
First you must always have the process's PID or its reference name.
Process.info/2
will give you information about that Process. You may get more documentation and information on how this function works in the Erlang's function it is calling:
process_info-2
There are also arity 1 variants: Process Docs
[erlang:process_info(Pid, initial_call) || Pid <- erlang:processes()].
But note that gen_server, etc., all have the same initial call, so you need to dig a little deeper.
The following is adapted from https://gist.github.com/rlipscombe/a8e87583d47799170f8b:
lists:map(
fun(Pid) ->
InitialCall = case erlang:process_info(Pid, initial_call) of
{initial_call,{proc_lib,init_p,A}} ->
case erlang:process_info(Pid, dictionary) of
{dictionary, D} ->
proplists:get_value('$initial_call', D, undefined);
_ ->
{proc_lib,init_p,A}
end;
{initial_call,{erlang,apply,A}} ->
case erlang:process_info(Pid, current_function) of
{current_function,MFA} -> MFA;
_ -> {erlang,apply,A}
end;
{initial_call,IC} ->
IC;
Other ->
Other
end,
{Pid, InitialCall}
end, erlang:processes()).
Using process_info/1 you can get a list of process information from which initial_call and current_function could help you to find the initial function call with which the process was spawned and the current function call of the process respectively.
Also process_info(Pid, initial_call) and process_info(Pid, current_function) functions are using as shortcut.

Writing and compiling custom behaviours in Erlang

I'm trying to write and compile a custom behaviour in Erlang.
I cannot find any clear documentation on how to compile this behaviour.
-module(bla).
-export([start_link/0,behaviour_info/1]).
behaviour_info(callbacks)->
[{init,1}];
behaviour_info(_Other)->
undefined.
%% -callback init(Args :: term()) ->
%% {ok, State :: term()} | {ok, State :: term(), timeout()} |
%% {stop, Reason :: term()} | ignore.
start_link()->
init([]).
my command for comiling is :
erlc.exe .\src\bla.erl
resulting:
bla.erl:24: function init/1 undefined
Anyone an idea on writing and compiling behaviours in erlang, please? any links?
Defining behaviour callbacks results in an obligation towards your implementation of callback module. In erlang, modules are just function containers, not classes nor interfaces. Behaviours are based on runtime module name resolution Mod:fun(). OTP gen_server (check it) keeps its callback module name after you pass it in: gen_server:start_link(CallbackModuleName, Args, Opts) and here is code for applying callback init/1:
init_it(Starter, Parent, Name0, Mod, Args, Options) ->
Name = name(Name0),
Debug = debug_options(Name, Options),
case catch Mod:init(Args) of
{ok, State} ->
proc_lib:init_ack(Starter, {ok, self()}),
loop(Parent, Name, State, Mod, infinity, Debug);
{ok, State, Timeout} ->
proc_lib:init_ack(Starter, {ok, self()}),
loop(Parent, Name, State, Mod, Timeout, Debug);
...
It's applying init/1 of passed callback module kept in Mod parameter, gets its last value, do what you want and keep going (or not, depends on that last value).
Assume we have module bla_impl which looks like this:
-module(bla_impl).
-behaviour(bla).
-export([init/1, start_link/0]).
start_link() -> bla:start_link(?MODULE). %% macro ?MODULE is resolved to bla_impl
init(Args) -> ... .
And now you need to say in bla which module you use by:
-module(bla).
-export([start_link/1]).
start_link(Mod) -> Mod:init([]).
or maybe better solution is to read it from configuration:
-module(bla).
-export([start_link/0]).
start_link() ->
Mod = application:get_env(bla_app, callback_module),
Mod:init([]),
...
There is many ways for doing so.
As you see there is no magic here. This would work even without -behaviour(bla) nor specified callback with -callback. This is just an information for compiler, tools and documentation.
From erlang documentation: Behaviours
And btw. start_link function should spawn another process and link to it.
start_link(Mod) ->
spawn_link(Mod, init, [[]]).

How to test gen_server internal state with eunit

Is it possible to inspect the internal state of a gen_server after a callback function has been called? I would rather not change the API of my server here.
You could use sys:get_state/1 which works nicely with all gen's.
Maybe, you would found useful another approach to unit-testing gen_servers.
Instead of running gen_server process and testing its behaviour, you can directly test gen_server callbacks and then inspect its state transitions.
For example:
-module(foo_server).
%% Some code skipped
handle_call({do_stuf, Arg}, _From, State) ->
NewState = modify_state(
{reply, {stuf_done, Arg}, NewState}.
%% Some code skipped
-ifdef(TEST)
do_stuf_test_() ->
{setup,
fun() ->
{ok, InitState} = foo_server:init(SomeInitParams),
InitState
end,
fun(State) ->
ok = foo_server:terminate(shutdown, State)
end,
fun(State) ->
Result = foo_server:handle_call({do_stuf, hello}, undefined, State),
[
?_assertMatch({reply, {stuf_done, hello}, _}, Result)
]
end
}
}.
-endif.
See discussion of this approach here
Also, if you dealing with realy complex states and state transitions, maybe you would be found proper helpful.

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